1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation having a luminescent storage screen for the latent storage of an x-ray image, and in particular to a read-out system for such an installation.
2. Description of the Prior Art
It is known to store a latent image generated by attenuated x-radiation using a luminescent storage screen. To read-out the latent image from the storage screen, the storage screen is caused to luminesce pixel-by-pixel by planar scanning with a scanning beam of read-out radiation. The light emitted in this manner by the luminescent storage screen is acquired by a light detector, with the collected light being converted into electrical signals, from which a visible image is constructed.
A read-out system of the type described above is described in U.S. Pat. No. 4,629,890, wherein a tubular light guide, having interior walls which are mirrored, is disposed over the luminescent storage screen which is to be read. The tubular light guide has a slot-shaped passage through which the scan beam, which is deflected within a plane by a rotatable mirror, can pass, so that a complete line of the storage screen is scanned and thereby excited to luminesce. As a result, the storage screen emits light pixel-by-pixel, which is reflected by the walls of the tubular light guide, and is conducted onto the input surfaces of two detectors, disposed side-by-side. The detectors generate respective electrical signals corresponding to the brightness of the light emitted by each pixel of the storage screen.
In a light guide of this type, the tube diameter cannot be excessively small, otherwise too many reflections, which excessively attenuate the light, will occur. The light which is incident on the end faces of the light guide must also be conducted with good efficiency.
Photomultipliers are known which have laterally disposed input windows, however, these types of photomultiplier have so-called incident light cathodes, i.e., the cathode plate is arranged at a distance behind the input window, so that the electrons emitted in the forward direction can be extracted as part of the output signal. In order for the major portion of the light emerging from the tubular light guide to be acquired, photomultipliers having lateral windows considerably larger in dimension than the optimum tube diameter would be required. There are no commercially available photomultipliers having a lateral window with such large dimensions.
Another type of photomultiplier uses a so-called transillumination cathode, which need only have a diameter of the size of the diameter of the photomultiplier tube, because the photo cathode in such photomultipliers is situated substantially at the tube ends. This produces an extremely bulky structure, because photomultipliers are flanged along the tube axis. A sensitivity curve which favors (i.e., is more sensitive at) the lateral regions of the storage screen results, due to the lateral arrangement of the detectors, because the light path at these regions between the storage screen and the detector is more direct, i.e., has fewer reflections. The most important image information, however, is usually in the central portion of the image, with the result being that this portion of the image is attenuated in comparison to the edge regions.